1. Field of the Invention disc
This invention relates to a lift check valve and, more specifically, to such a check valve which may be utilized in the discharge line of a feedwater pump for a nuclear power system.
2. Description of the Prior Art
Heretofore, lift check valves similar to that disclosed in U.S. Pat. No. 2,665,877 have been utilized on the discharge side of feedwater pumps in nuclear power plants to prevent backflow through a non-operating pump as might occur when other pumps in the system are being operated. The check valve is opened by normal flow from the pump and will close when flow discontinues. With the cessation of normal flow, gravity acts on the disc to lower it into the feedwater stream where any reverse flow causes the disc to fully seat. An automatic, self-operating valve is desirable whenever possible in a feedwater system of a nuclear power plant because it is selfcontained and does not require the pressure boundary to be penetrated. This decreases the likelihood of leakage and eliminates the need for packing which an externally operated valve might require.
As seen in U.S. Pat. No. 2,665,877, as well as U.S. Pat. No. 2,688,980, which are incorporated herein by reference, an equalizing line is utilized in the feedwater system to insure that the valve is fully withdrawn from the pump discharge flow path during pump operation. Specifically, the equalizing line communicates with the downstream side of the valve where it is subjected to a lower pressure than that acting on the valve disc because of fluid pressure drops through the line. With the other end of the equalizing line communicating with the region above the valve, the resulting differential pressure acting on the valve overcomes gravitational forces on the valve and causes it to be fully withdrawn from the flow stream during expected normal flow rates.
With an increased emphasis on safety and reliability of all features of nuclear power plant systems, consideration has been given to the effect of an instantaneous, complete rupture of the feedwater pipe between the pump and the lift check valve. A mathematical analysis, using accepted engineering and analytical methods, has indicated that a lift check valve similar to that disclosed in U.S. Pat. No. 2,665,877 would not operate satisfactorily under such a demanding condition. For the purposes of this analysis, it was assumed that the feedwater system included water at 1,250 p.s.i. and 420.degree. F. with a flow rate of 5,220,000 pounds per hour away from the impending break. This flow rate was determined to be about 23 feet per second through a 151/2 inches inside diameter line in which an 18 inches lift check valve would be utilized. The valve utilized in the analysis was somewhat different from the valve of U.S. Pat. No. 2,665,877, however, in that it had an operating chamber which was oriented at a 45.degree. angle (as will be obvious from the preferred embodiment of the invention as shown in FIG. 1) with respect to the feedwater line in which it was installed. The calculations took into effect such items as the fact that the water could be expected to progressively turn into saturated steam at about 300 p.s.i. after the rupture and generally assumed the most favorable parameters for valve operation.
It was still found, however, that excessive seat impact and excessive water hammer surge pressures would result. Specifically, the valve of the above mentioned analysis would produce severe plastic deformation of the seat as a result of a seating velocity of 90 feet per second and a surge pressure in the feedwater line of over 1,000 p.s.i., in addition to the 1,250 p.s.i. operating pressure of the feedwater. The chamfered seat of the valve, for example, might be plastically deformed axially as much as one-fourth of an inch to prevent a satisfactory sealed closure after the rupture. The excessive surge pressure could have a detrimental effect on any number of components within the system and might, for example, overload the pipe supports and cause their failure.
Since it was apparent that a simple lift check valve would not satisfactorily withstand such a criteria, the use of an alternative, existing valve was considered. For example, it was felt that an externally operated valve such as disclosed in U.S. Pat. No. 3,888,280 and incorporated herein by reference, could be employed. The valve disclosed therein would be automatically closed by activation of its external controls when an associated sensing system sensed a loss of feedwater pressure. However, unless modified, this valve is intended to require three to five seconds for complete closure allowing a significant loss of feedwater. Additionally, there are a number of features of the valve of U.S. Pat. No. 3,888,280 which make it unattractive as an alternative to the prior feed pump check valve. Since the valve is an externally operated valve, it includes a significant number of elements which should not be needed for a check valve. The valve is relatively expensive, requires that the pressure boundary be penetrated and generally requires additional space which complicates system design.